To reproduce, add <distributable> to a webapp's web.xml and deploy to a default server profile. You'll see the following exception thrown on the server console:
11:50:38,181 ERROR [JBossCacheManager] JBossCacheService to Tomcat clustering not found
followed by ..
11:50:38,215 ERROR [TomcatDeployer] Failed to setup clustering, clustering disabled. Exception:
org.jboss.web.tomcat.tc5.session.ClusteringNotSupportedException: JBossCacheService to Tomcat clustering not found
While the exception is true and the cluster is disabled at the end. It should only warn the user and not throw the exception.